The Living World: Biodiversity

The Living World: Biodiversity (Unit 2)

Introduction to Biodiversity (Module 8)

The Benefits of Biodiversity: A Case Study

  • Biodiversity Defined: The diversity of life forms in an environment, a combination of "biological" and "diversity."
  • Historical Appreciation of Biodiversity: Species in nature have long been recognized for providing essential benefits to humans, including food, fiber, lumber, and medicines.
  • Ecosystem Contributions: Collections of species within ecosystems provide vital services such as:
    • Primary productivity in land and water.
    • Production of clean water.
    • Buffers against natural disasters.
  • Central Question: Does humanity's ability to enjoy these ecosystem benefits critically depend on maintaining historical levels of biodiversity, or can we sustain them even if biodiversity declines due to human activities?

Scientific Debate and Findings

  • The Debate (Three Decades): Scientists have debated whether greater biodiversity yields greater ecosystem benefits.
  • Experimental Results (Aquatic and Terrestrial):
    • Food webs with a greater number of species consistently show higher overall productivity.
    • Ecosystems with higher species diversity are more stable, especially when the abiotic environment changes.
    • Example (Drought): An ecosystem with higher species diversity can recover more quickly after a drought ends and rain returns, compared to an ecosystem with low species diversity.
  • Criticisms of Experiments:
    • Many experiments contain significantly fewer species than exist in natural environments, raising questions about the applicability of results to nature.
    • It's possible that the productivity of natural ecosystems is more heavily influenced by variations in climatic conditions and available nutrients than by species richness, potentially making experimental results "trivial" in the real world.
  • Real-World Research (67 Studies):
    • Researchers compiled data from 67 studies that measured species number and ecosystem productivity across 600,000 sample locations worldwide.
    • Consistent Finding: Across terrestrial, freshwater, and marine biomes, sites with more species consistently exhibited higher productivity, aligning with experimental results.
    • Comparative Importance: When climatic conditions and available nutrients were considered, biodiversity was found to be:
      • A more important factor than climate in half of the studies.
      • A more important factor than nutrients in two-thirds of the studies.
  • Conclusion: Biodiversity is critical to the proper functioning of ecosystems, underscoring the necessity to protect Earth's biodiversity to maintain its numerous benefits.

Unit 2 Overview: The Living World: Biodiversity

  • Purpose: To examine biodiversity in depth, from genes to ecosystems.
  • Key Topics:
    • How diversity allows ecosystems to provide important services.
    • How species numbers are influenced by colonization and extinction due to suitable/unsuitable conditions.
    • How ecological disruptions (short-term and long-term) alter species composition.
    • The evolution of species traits through natural selection to persist in ecosystems.
    • Ecological succession: Initial species being replaced by better-suited species over time.

Biodiversity as an Environmental Indicator

  • Biodiversity serves as a crucial indicator of environmental health. A rapid decline signals an ecosystem under stress.
  • This module explores how scientists define and quantify biodiversity at different scales and estimates the total number of species on Earth.

8-1 How does biodiversity exist at different scales?

  • Biodiversity exists on four interconnected scales, each reflecting environmental health:
    • Genetic diversity
    • Species diversity
    • Habitat diversity
    • Ecosystem diversity
Genetic Diversity
  • Definition: A measure of the genetic variation among individuals within a population.
  • Characteristics: Large populations generally possess high genetic composition variation.
  • Example (White-tailed Deer - Odocoileus virginianus):
    • Distributed across North, Central, and northern South America, inhabiting diverse habitats (fields, forests, swamps).
    • Regional adaptations: Deer in colder climates evolved larger bodies to conserve heat; deer in warm climates (e.g., Florida) evolved smaller bodies to dissipate heat easily.
  • Population Bottleneck:
    • Definition: A significant reduction in genetic diversity within a population when a large population declines, and only a small percentage of individuals survive, carrying a limited amount of the original genetic variation.
    • Consequence: Even if the population rebounds, it retains low genetic diversity.
    • Example (Cheetah - Acinonyx jubatus):
      • Experienced a population bottleneck approximately 10,000 years ago, leading to today's low genetic variation.
      • Implication: This low genetic diversity makes cheetahs highly susceptible to pathogens; one pathogen kills up to 70 percent of captive cheetahs.
Species Diversity
  • Species Definition (from Module 0): A group of organisms distinct from other groups in morphology, behavior, or biochemical properties. Individuals of the same species can breed and produce viable and fertile offspring.
    • Note: Individuals from different species may mate but typically produce non-surviving or infertile offspring.
  • Species Diversity Definition: The number of species in a given region or particular ecosystem.
  • Examples of Variation:
    • Low Diversity: Tundra and taiga biomes (plants and animals), commercial timber plantations (typically 1 or a few tree species).
    • High Diversity: Tropical rainforests (very high species diversity), temperate deciduous forests (dozens of tree and shrub species).
Habitat Diversity
  • Definition: The variety of habitats that exist within a given ecosystem.
  • Species Types related to Habitat:
    • Specialists: Species that can only live under a narrow range of biotic or abiotic conditions.
      • Example: Koala (Phascolarctos cinereus) feeding exclusively on eucalyptus leaves and living among eucalyptus trees.
    • Generalists: Species that can live under a wide range of biotic or abiotic conditions.
      • Example: White-tailed deer, adaptable to various climates and feeding on many different plants.
  • Impact of Habitat Diversity: A greater diversity of habitats supports a greater diversity of species because different habitats accommodate unique suites of specialist and generalist species.
    • Example: A region with mixed farms (crops, pastures), abandoned farms (shrubs), and forests (large trees) supports many more species than a homogeneous environment.
  • Example (Vermont Stream Birds):
    • 27 streams supplying Lake Champlain averaged only 17 bird species each, but the entire collection supported 101 species.
    • Reason: Different bird types preferred specific habitats:
      • Fish-eating birds: deeper streams with minimal agriculture.
      • Wading birds: shallow streams.
      • Insect-eating birds: streams surrounded by meadows and forests.
    • Conclusion: A diverse suite of habitats is crucial for maintaining high species diversity.
Ecosystem Diversity
  • Definition: The variety of ecosystems that exist in a given region (the largest scale of biodiversity).
  • Impact: A greater diversity of ecosystems allows for more species to live in a given area.
  • Examples:
    • Coasts of China: Mixture of temperate forest, river, and ocean ecosystems.
    • Los Angeles County, California: Highest bird diversity in the U.S. due to numerous diverse habitats including oak woodlands, chaparral, deserts, urban environments, and the ocean.

Do the Math: Converting Between Hectares and Acres

  • Land Area Conversions:
    • United States: Square miles or acres.
      • 1 ext{ square mile} = 640 ext{ acres}
      • 1 ext{ acre} ext{ approximately } = 64 ext{ m} imes 64 ext{ m} (or 209 ext{ feet} imes 209 ext{ feet})
    • Rest of the World: Hectares ( ext{ha}).
      • 1 ext{ ha} = 100 ext{ m} imes 100 ext{ m}
      • 1 ext{ ha} = 2.47 ext{ acres} (for quick estimation, round to 2.5 ext{ acres})
  • Example Calculation: A nature preserve of 100 ext{ ha}.
    • 100 ext{ ha} imes 2.5 ext{ acres/ha} = 250 ext{ acres}
  • Your Turn (Calculation): A forest is 10,000 ext{ acres}. Its size in hectares is:
    • 10,000 ext{ acres} / 2.5 ext{ acres/ha} = 4,000 ext{ ha}

8-2 How does biodiversity affect responses to environmental stressors?

  • Increased biodiversity enhances the ability of populations and ecosystems to respond to biotic or abiotic environmental stressors.
Consequences of Genetic Diversity
  • Benefit of High Genetic Diversity: Improves the long-term persistence of populations by enabling better responses to environmental change.
    • Example (Fish Disease Resistance): A fish population with high genetic diversity for disease resistance is more likely to have some individuals carrying genes that allow them to survive a disease outbreak.
    • Risk of Low Genetic Diversity: Increases the likelihood of significant population decline when exposed to disease or other stressors.
    • Example (Cheetahs): Their low genetic diversity makes them highly vulnerable to pathogens, with one specific pathogen capable of killing up to 70 percent of captive cheetahs.
  • Importance in Domesticated Species:
    • Agricultural Crops: Centuries of breeding created hundreds of varieties (e.g., corn, rice, wheat).
      • When a new disease emerges, researchers can find resistant genes in other varieties, leveraging genetic diversity.
    • Concern: Environmental scientists are concerned about the rapid decline in genetically distinct varieties of domesticated plants and animals over the past century.
      • Farmers prioritize a small number of highly productive varieties, leading to the extinction of many others.
      • Example (Apples): In the 1900 ext{s}, U.S. farmers grew 8,000 varieties; today, over 95 ext{%} of these varieties are extinct.
Consequences of Habitat Diversity for Specialist and Generalist Species
  • Effect of High Habitat Diversity: Leads to an increase in the total number of species, as each habitat supports unique specialists and multiple habitats support shared generalists.
  • Impact of Habitat Loss (due to human activities):
    • Loss of a Single Habitat Type: Primarily results in the loss of specialist species confined to that habitat. Generalists may persist if other suitable habitats remain.
    • Destruction of Multiple Habitats: Eventually leads to the loss of generalist species as well.
    • Impact on Wide-Ranging Species: Habitat loss can severely reduce populations of species requiring large, continuous habitats (e.g., migrating buffalo herds, large predators like wolves and mountain lions).
Consequences of Species Diversity
  • Benefits of Higher Species Diversity: Results in ecosystems that are more productive and resilient.
    • Increased Primary Productivity: Greater rates of energy capture by producers.
    • Enhanced Resilience: Better ability to resist the impacts of disturbances (e.g., drought, hurricanes, fires) or to recover rapidly from them.
  • Experimental Evidence (Soil Fungi and Plant Growth):
    • Mutualism: Soil fungi live near plant roots, providing phosphorus to plants in exchange for sugars from photosynthesis.
    • Experiment: Manipulating the number of fungal species in soil revealed that increased fungal diversity led to increased plant growth, specifically:
      • An increase in the total amount of phosphorus found in the plants.
      • An increase in the biomass of plant roots.
      • An increase in the biomass of plant shoots (stems and leaves).
  • Experimental Evidence (Grassland Ecosystem Stability):
    • Experiment: Manipulated the number of grassland plant species in Minnesota and monitored herbivores, predators, and parasites over 11 years, encompassing environmental changes (droughts, abundant rainfall).
    • Findings: Increasing the number of plant species led to greater stability in the numbers of herbivore species and predator/parasite species.
  • Species Diversity as an Environmental Indicator:
    • Environmental scientists often use species diversity as a critical indicator.
    • Example (Frogs): The number of frog species is used as an indicator of regional environmental health because frogs are exposed to both water and air in their ecosystems. A decrease can signal problems at local and larger scales.

8-3 How can we calculate biodiversity?

  • Biodiversity can be quantified using two main measures:
    • Species Richness: The number of different species in a given area (e.g., a pond, a forest canopy, a grassland plot). It provides an approximate sense of diversity.
    • Species Evenness: The relative proportion of individuals within the different species in a given area. It indicates whether an ecosystem is dominated by a few species or if all species have similar abundances.
  • Interpreting Richness and Evenness:
    • High Species Evenness: Occurs when all species in an ecosystem are represented by similar numbers of individuals.
    • Low Species Evenness: Occurs when one or a few species are represented by many individuals, while others have only a few. A location with low species evenness is considered less diverse.
  • Example (Two Forest Communities - FIGURE 8.6):
    • Both Community 1 and Community 2 contain 20 trees distributed among 4 species.
    • Community 1: Each species is represented by 5 individuals (25 ext{%} each).
    • Community 2: One species has 14 individuals (70 ext{%}), and the other three each have 2 individuals (10 ext{%} each).
    • Conclusion: Both have identical species richness (4 species), but Community 1 has greater species evenness and is therefore considered more diverse.
  • Baseline Importance: Knowing both species richness and evenness provides environmental scientists with a baseline to assess how much an ecosystem has changed following human disturbance (as both often decline).

8-4 How can we estimate the number of species living on Earth?

  • Estimating the total number of species is challenging due to species being difficult to find (nocturnal, inaccessible locations like the deep ocean, microscopic).
  • Currently Described Species: Scientists have named approximately 2 million species to date.
  • Rate of Discovery: A staggering 15,000 ext{ to } 18,000 new species are discovered annually, highlighting the vast unknown biodiversity.
  • Insect Estimation Strategy (Historically):
    • Rationale: Insects comprise more species than most other groups, making their estimation key to understanding overall species numbers.
    • Method: Researchers fumigated canopies of a single tropical rainforest tree species, collected falling dead insects, and counted beetle species exclusively feeding on that tree.
    • Initial Calculation: Multiplied the number of beetle species per tree by the total number of tropical tree species, estimating 8 million beetle species feeding on individual tree species in the tropics.
    • Extrapolation (Assumptions):
      • Beetles constitute around 40 ext{%} of all insect species.
      • Insect species in the forest canopy are roughly twice as numerous as those on the forest floor.
    • Result: Suggested an initial estimate of 30 million total tropical insect species.
    • Revised View: More recent work indicates this estimate is likely too high.
  • Current Total Species Estimates: Range from 5 million to 100 million.
  • Most Accepted Scientific Estimate: Approximately 10 million total species on Earth.
  • Module Conclusion: Emphasizes biodiversity's crucial role in providing resilience from genetic to ecosystem levels.

Ecosystem Services (Module 9)

  • Context: Building on the importance of biodiversity and its role in ecosystem resilience.
  • Central Question: Are Earth's natural life-support systems being degraded by human activities?
  • Ecosystem Services Definition: The processes by which essential life-supporting resources are produced.
    • Examples: Clean water, timber, fisheries, agricultural crops.
  • Benefits:
    • Direct: Providing immediate necessities like food and drinking water.
    • Indirect: Creating diverse conditions, nutrients, and species that enhance the overall health of the ecosystem for all organisms, including humans.
  • Visibility of Degradation: Healthy ecosystems are often taken for granted. Their degradation becomes apparent when they can no longer provide services (e.g., water purification) or produce goods (e.g., fruit, firewood) as before.